It's not ATC which makes such regulations or even asks for them and a good few controllers I worked with did not like 2.5nm spacing. ATC received no specific training - 2.5nm was introduced when I worked and that's going back many years. Major airports are commercial organisations and the airlines demand space for more and more flights to land at them. One answer is to put the aircraft closer together. Who determines finally that this shall be done I do not know (Gonzo might) but one day I arrived at work and was told that 2.5nm may be used under certain conditions so it was a case of sit down and get on with it.